Displaying multiple render targets on one widget

HI Guys, I’m new to using Render Targets and slate brushes to display cameras as opposed to just using the output from a regular camera and was hoping someone could give me some guidance on the subject.

Right now I’m creating a custom first person view that uses two cameras, both cameras are at the same position and capture the same FOV but I want to use one to display what should be in the white circle and the other to display everything outside of that. I want the center circle to be in colour and high definition and the out side I want to be grayscale and fuzzy. I already know the camera settings I need to change to achieve the look I want so my question is how do I use slate brushes to paint onto an image so that one texture only fills in the circle and the other fills in the outside?

I tried importing an image of a circle with a transparent background and brushing the texture onto that but the texture fills out the square borders of the image not just the circle itself. If anyone knows or has a different solution to achieve the effect I’m going for any advice or criticism would be greatly appreciated .